The present invention generally relates to a carburetor system for an internal combustion engine, and more specifically to a carburetor system having means for adjusting the air fuel ratio by additionally introducing air to a main fuel system so as to compensate for altitude change or other changes of engine operating conditions.
In such a carburetor system, there is formed an auxiliary air introduction passage for additionally supplying air to a main fuel system and a valve device for controlling the air flow through the auxiliary air passage in accordance with atmospheric pressure, ambient air temperature or other variables. An upstream end of the auxiliary air passage communicates with the atmosphere and its downstream end opens into a main fuel passage at a portion where an air bleed pipe is inserted. The downstream end of the auxillary air passage is positioned below a fuel level, so that a fuel normally fills a portion of the auxiliary air passage adjacent to the downstream end to the fuel level. In some cases, fuel is sucked into the auxiliary air passage beyond the fuel level because of a pressure difference between the main fuel system and the auxiliary air passage. Occasionally, fuel contained in the auxiliary air passage is abruptly forced out into the main fuel system under the influence of rapidly changing engine operating conditions. This fuel is supplied to the engine and causes an abrupt enrichment of the air fuel mixture, unfavorably affecting engine drivability and emission control.